Production of dicarboxylic acids

ABSTRACT

Dicarboxylic acids or esters thereof are recovered from solid phase polyester materials, such as post-consumer products and factory scrap, by subjecting the polyester to at least two hydrolysis stages in at least the first of which the amount of water used is substantially less than needed to effect total conversion of the polyester to the dicarboxylic acid. Also the diol content is controlled in the course of carrying out the hydrolysis. The hydrolysis reactions may be preceded by reaction of the polyester with a diol, the resulting depolymerisation products then being hydrolysed.

This invention relates to the production of dicarboxylic acids.

The invention is particularly concerned with the production of suchacids by means of the depolymerisation of polyesters, particularlycondensation polyesters such as polyalkylene terephthalates andpolyalkylene naphthalates, in order to recover dicarboxylic acids.

It is known that terephthalic acid which is suitable for polymerisationwith alkylene glycols either directly or after purification may beobtained by the hydrolysis of waste polyalkylene terephthalate.GB-A-2123403 discloses a continuous procedure for obtaining terephthalicacid from PET waste in which the PET waste is heated in water and inwhich process the presence of decolourising carbon in the water isessential. Additionally, this procedure utilises sufficient water, andis operated at such a temperature, that the terephthalic acid productdissolves in the water as it is produced to form an aqueous solution ofterephthalic acid, there being substantially no terephthalic acid in thesolid phase, which solution is subsequently filtered to remove thecarbon: the terephthalic acid is then crystallised from the filtrate.

It is also known from East German Patent No. 14854 to produceterephthalic acid by hydrolysis of PET. In this case, the teaching alsoappears to be directed towards producing the terephthalic acid productin solution at the reaction conditions employed. The disclosure refersto filtering the hot reaction solution under pressure using a filterwhich can trap both coloring and mechanical impurities. The hot solutionis thereafter cooled to crystallise the terephthalic acid which is thenisolated and dried.

U.S. Pat. No. 5095145 is likewise concerned with effectingdepolymerisation of waste PET products by depolymerisation thereof in anaqueous mixture at a temperature within the range 430° to 600° F. toproduce an aqueous crude terephthalic acid solution which is thereafterprocessed further.

U.S. Pat. No. 3257335 discloses a two stage process for depolymerisingpolyesters, particularly polyethylene terephthalate, to produce lowmolecular weight terephthalic esters of ethylene glycol in ethyleneglycol solutions which can be stored as a liquid at reduced temperaturesfor extended periods of time without solidification or excessivedegradation. The process disclosed comprises dissolving waste polyesterin monomer at atmospheric pressure and at a temperature greater than theboiling point of ethylene glycol but less that the boiling point of themixture, pumping the resulting solution together with fresh ethyleneglycol into a tubular reactor at a higher temperature than the dissolverand pressure in excess of the vapor pressure of ethylene glycol at thattemperature, recycling part of the reaction product to the dissolver andremoving the remainder for storage.

In our prior European Patent Application No. 92 311421.9, there isdisclosed a process for the production of terephthalic acid frompolyalkylene terephthalate by hydrolysing the terephthalate in such away that, at the reaction temperature, at least part (preferably a majorpart) of the terephthalic acid is produced in the solid phase. Thisprovides benefits in terms of the extent of post-reactioncrystallisation necessary to recover the terephthalic acid and theextent of water removal necessary to effect recovery of glycol producedin the reaction. The hydrolysis may be carried out in two stages andglycol may be present in the reaction mixture so as to increase theproportion of glycol present relative to water thereby simplifying therecovery of glycol following the reaction.

The present invention seeks to provide an improved process for thedepolymerisation of polyesters.

According to the present invention there is provided a process foreffecting depolymerisation of polyesters in order to recover theconstituent dicarboxylic acid and diol therefrom, including the stepsof:

(a) subjecting a polyester, or a treated polyester as herein specified,to a first hydrolysis reaction in such a way that the polyester (ortreated polyester) is only partially converted to the constituent dioland the dicarboxylic acid;

(b) separating from the mixture formed in step (a) at least part of thediol present in step (a); and

(c) subjecting the mixture remaining after step (b) to at least onefurther hydrolysis reaction.

As referred to herein, "a pre-treated polyester" refers to a solid phasepolyester which, prior to said first stage hydrolysis, has been treatedto produce a liquid phase medium which contains primarily low molecularweight depolymerisation products (herein referred to as oligomers) andmay contain some higher molecular weight depolymerisation products (inexcess of 20 repeat units) and possibly some unreacted polyester.

The invention has application for example to polyesters such aspolyalkylene terephthalates in which the dicarboxylic acid comprisesterephthalic acid and polyalkylene naphthalates in which thedicarboxylic acid or ester comprises naphthylene 2,6 dicarboxylic acid.

Because the hydrolysis of polyester is an equilibrium process, it isnecessary to have a large excess of water over diol in the finalreaction mixture to obtain high yields of dicarboxylic acid and diols.Removing even a small amount of diol will thus reduce the requiredamount of water by a much greater amount. By carrying out the hydrolysisin more than one stage and removing the diol as in step (b) prior to theor each following hydrolysis stage, the invention makes it possible touse relatively high polyester (or treated polyester) loadings relativeto the total amount of water used in the process while still obtaininghigh conversion of polyester to acid and glycol. This in turn results inlesser amounts of effluent requiring treatment prior to disposal, lessdistillation to recover the diol and reduced operating costs.

The diol present in any hydrolysis reaction will be present in either orboth of two forms, free diol (normally dissolved in the aqueous phase)and unhydrolysed diol reacted with the diacid and present in theoligomers or polyester material. It has surprisingly been found that theuse of an intermediate hydrolysis step will liberate a significantportion of the diol present in oligomers without the need for largequantities of water. Once liberated, the free diol may be removed moreeasily.

Typically the process is carried out in two hydrolysis reaction stages.However, it may be carried out in three or even more hydrolysis reactionstages. Preferably in all but the final stage between 10 and 99% (morepreferably between 50 and 90%) of the diol which is combined at eachsuch stage with dicarboxylic acid species is liberated. In the finalstage preferably at least 90%, more preferably more than 95% andespecially more than 99%, of the remaining bound diol is liberated.

It is to be understood that the references to hydrolysis reaction stagesherein are not limited to each hydrolysis reaction being carried outseparately or in separate reactors. The invention includes within itsscope, the carrying out of one hydrolysis reaction concurrently with, iein the same reactor as, a preceding hydrolysis reaction. For instance,in the case of polyester or treated polyester material which hasundergone a first stage hydrolysis, the product of the reaction(following separation therefrom of at least part of the diol present inthe first stage hydrolysis reaction) may be recycled at least in partand combined with a "fresh" incoming (unhydrolysed) polyester/treatedpolyester in the same reactor and the combined materials subjected tohydrolysis. Thus, in this instance, the hydrolysis reaction performedwill constitute a second stage hydrolysis reaction for the recycledmaterial and a first stage hydrolysis reaction for the "fresh" incomingmaterial. More generally, where the process involves more than onehydrolysis reaction stage, it will be appreciated that material atdifferent stages of hydrolysis (including material which has yet to besubjected to a first stage hydrolysis) may be hydrolysed in the samereactor concurrently so that, while some material is undergoing its nthhydrolysis reaction stage, other material is undergoing its (n+m)thhydrolysis reaction stage, where n is an integer equal to one or moreand m is an integer usually equal to one. Moreover, the various stagesof hydrolysis may be carried out either continuously or in batchwisefashion, ie material feed to each hydrolysis being effected continuouslyor batchwise.

In practice, the first hydrolysis reaction stage is carried out using anamount of aqueous medium which is less than would otherwise be necessaryto effect substantially total conversion of the polyester to thepolycarboxylic acid.

Preferably said first stage hydrolysis reaction is preceded by treatmentof the polyester with a diol to convert it from a solid phase form tosaid liquid phase medium.

The liquid phase medium following the reaction of the polyester with thediol will also often contain residual diol, particularly where an excessof the diol is used in the reaction.

The hydrolysis reaction generally requires elevated pressure which givesrise to problems, especially in continuous processes, from thestandpoint of introducing the solid phase polyester into a hydrolysisreactor under elevated pressure and temperature conditions since solidphase polyester (eg post-consumer and factory scrap) is commonly in aform which is not readily amenable to slurrying and pumping. Also, solidphase often has a relatively low bulk density and the amount of aqueousmedium required to totally immerse the polyester is significantlygreater than that needed on a weight for weight basis to treat thatamount of polyester. By preceding the first stage hydrolysis withreaction of the solid phase polyester with a diol in order to producesaid liquid phase medium, the problem of introducing a low bulk densitysolid phase reactant at atmospheric pressure into elevated pressureconditions is obviated since it is a relatively simple matter to pump aliquid phase medium into a reactor operating under high pressureconditions. Also, the amount of water can be substantially reducedcompared with that required to totally immerse low bulk density solidphase polyester. Because diols are much less volatile than water, thereaction of the solid phase polyester with the diol can be carried outat least initially at much lower pressure, eg atmospheric pressure, thanis necessary for the first stage hydrolysis reaction.

Another advantage stemming from the preceding reaction with the diol isthat the solid phase polyester can be fed substantially continuouslyinto that reaction whereas continuous feed of solid phase polyesterdirectly into a hydrolysis reactor operating under elevated temperatureand pressure conditions is technically difficult.

A further advantage conferred by the preceding reaction with the diol,as opposed to carrying out the hydrolysis of solid phase polyesterdirectly, is that various treatments of the resulting liquid phasemedium can be carried out prior to carrying out the first stagehydrolysis reaction. In particular, it becomes feasible to subject theliquid phase medium to a separation process, eg filtration, to removeundesirable insoluble particulate impurities, such as aluminum, paper,polystyrene, polyolefines, PVC, commonly present in washed,scrap/post-consumer polyesters. Also, by suitable management of theliquid phase medium, extraction of impurities by means of contact withfor instance activated carbon (eg for decolourising the liquid phasemedium) or an ion exchange resin may be effected in order to removecontaminants such as chloride or ionic catalyst residues present in theliquefied polyester.

Typically, the reaction of the solid phase polyester with the diol willbe carried out at elevated temperature in excess of temperatures atwhich use of ion exchange resins can normally be entertained; thus, forinstance, prior to extraction of contaminants using means such as an ionexchange resin, the liquid phase medium may be cooled to a temperaturewithin a range allowing the use of such extraction means. Water or othersuitable polar solvent may be added to the liquid phase medium to assistin solubilising the oligomers at lower temperatures and to assist inionising impurities to facilitate removal of soluble impurities.

The reaction of the solid phase polyester with the diol may be carriedout in two stages, a first low pressure stage in which the reaction withthe diol serves to produce said lower molecular weight polyesterdepolymerisation products and a second higher pressure stage in whichthe reaction with the diol proceeds further in order to produce evenlower molecular weight depolymerisation products. The second stage mayinvolve the introduction of additional diol (which is preferably thesame as that used in the first stage). The second stage confers theadvantage that the production of even lower molecular weight oligomerspermits the liquid phase to be cooled without solidifying totemperatures lower than is possible with longer chain polyesterdepolymerisation products. It is therefore more feasible to cool theliquid phase medium to within a temperature range compatible with theuse of ion exchange resins. Cooling may also be desirable where otherfiltration/removal techniques, eg activated carbon, semi-permeablemembranes, etc, are-used.

A further advantage of being able to cool to relatively low temperaturewithout solidification is that the liquid product can be stored as suchwithout degrading significantly.

The first stage of the polyester/diol reaction is conveniently carriedout in a continuous fashion with solid phase polyester being introducedto the reaction concurrently with removal of said liquid phase medium.Thus, in steady state operation of the process, liquid phase medium maybe continuously withdrawn from the first stage of the reaction and maybe pumped continuously into the higher pressure second stage of thepolyester/diol reaction.

Advantageously, particularly in terms of aiding eventual separation andrecovery thereof, the diol used in the first stage, and where thecontext admits in each stage, of the polyester/diol reaction is the sameas the diol which is derived from the subsequent hydrolysis reactionpolyester. For instance, in the case where the starting polyester isconstituted by polyethylene terephthalate, the alklene diol used in thepolyester/diol reaction (whether carried out in a single or multiplestages) is preferably monoethylene glycol. In some instances, thepolyester/diol reaction may be carried out using a mixture of differentalkylene diols of which one will preferably be the same as that derivedfrom the polyester in the subsequent hydrolysis reaction.

Some polyesters are produced by the reaction of a polycarboxylic acidwith a mixture of diols. Where the polyester to be processed inaccordance with the invention is of this type, the diol used in thepolyester/diol reaction is preferably the same as the diol which formedthe major component of the diol mixture originally used in theproduction of the polyester.

Preferably in at least the first stage hydrolysis, the polyester/treatedpolyester content relative to the water content of the aqueous mediumand the reaction conditions are such that, were 90% of the theoreticalTA based on the polyester/treated polyester initially present to beconverted to free dicarboxylic acid, then the solubility limit of thedicarboxylic acid would be exceeded and part of the acid wouldprecipitate under the reaction conditions.

Stated more specifically, preferably at least one of the hydrolysisreaction stages (usually at least the first hydrolysis reaction stage)is operated with a polyester or treated polyester loading (relative tothe aqueous phase used in the reaction) together with an aqueous phasecomposition such that one or both of the following conditions are met:

(a) the quantity of liquor is insufficient, under the reactionconditions prevailing during hydrolysis, to prevent the dicarboxylicacid produced exceeding its solubility limit in the liquor remainingfollowing the hydrolysis reaction; and

(b) the conversion of polyester or treated polyester to the acid andglycol is less than 90% (defined in terms of the % of the total acidavailable for hydrolysis which is actually produced as dicarboxylicacid).

Usually in a hydrolysis reaction stage where the total yield of freedicarboxylic acid is in excess of 40% based on the total amount ofdicarboxylic acid species fed to that stage, at least 20% (morepreferably at least 30% and especially at least 50%) of the freedicarboxylic acid is produced in the solid phase under the reactionconditions.

Often the penultimate hydrolysis reaction stage is carried out in such away that, under the prevailing reaction conditions, at least 20% (morepreferably at least 50%, especially at least 70%) of the dicarboxylicacid is produced in the solid phase.

Also the final hydrolysis reaction stage is preferably carried out insuch a way that, under the prevailing reaction conditions, at least amajor part (more preferably at least 70%, and most preferably at least80%, even as much as 90% or more) of the dicarboxylic acid is producedin the solid phase.

This can be achieved by employing comparatively low reaction temperaturefor the hydrolysis (certainly less than 300° C., typically in the range190° to 240° C. and preferably about 200° to about 220° C.) and by usingsmall quantities of water to secure high conversion to dicarboxylicacid. Hydrolysis at temperatures lower than 190° C. may be employed ifdesired; however in order to secure a dicarboxylic acid yield similar tothat obtained at higher temperatures, the reaction time must beincreased significantly.

A number of advantages may be secured by effecting the final hydrolysisreaction stage in such a way as to produce a substantial part of thedicarboxylic acid in the solid phase rather than being completelydissolved in the reaction medium. Thus, less recrystallisation isnecessary to recover the dicarboxylic acid from the reaction medium ascompared with the processes of the prior art.

Also, by ensuring that at least part of the dicarboxylic acid isproduced in the solid phase during the course of the final hydrolysisreaction, the reaction equilibrium can be shifted in the desireddirection thereby enhancing recovery of the dicarboxylic acid.

The solid phase polyester to be treated in accordance with the processof the invention may be in any suitable form although it is preferredthat the polyester is in the form of particles such as granules, powderor flakes, derived by the comminution or other mechanical breakdown ofmanufactured articles consisting of or containing polyester. Forinstance, in the case of polyethylene terephthalate (PET), bottlesprovide a major source of PET suitable for recycling to produceterephthalic acid although it may be desirable to separate the PET fromany other plastics materials contained in the bottles such aspolyvinylchloride (PVC) prior to hydrolysis. Other sources of PETinclude fibers and film.

Although it is preferred to comminute polyester products such as bottlesinto particles, flakes or other finely divided form we do not excludethe possibility of using the process of the invention to treat finishedpolyester products in order to recover the dicarboxylic acid.

The hydrolysis reaction stages are preferably carried out usingdemineralised water as the aqueous medium thus reducing the possibilityof competing reactions reducing the yield of terephthalic acid.

A further aspect of the present invention is concerned with effectinghydrolysis reaction in all but the final stage hydrolysis reaction insuch a way as to simplify removal of the diol produced from intermediatereaction products.

Thus, according to a further aspect of the invention there is provided aprocess for effecting depolymerisation of polyesters in order to recovera dicarboxylic acid and diol therefrom, including the steps of:

(i) subjecting a polyester, or a treated polyester as herein specified,to a first stage hydrolysis reaction at elevated temperature to convertat least the major proportion of the polyester (or treated polyester) tothe dicarboxylic acid and intermediate reaction products of the acidwith the diol, the reaction being carried out in such a way that, afterprecipitating at least a major proportion (and preferably substantiallyall) of the dissolved intermediate reaction products, the resultingreaction mixture is capable of separating into two distinct phases,namely a supernatant liquid phase substantially comprising water andsaid diol, and a solid phase comprising said intermediate reactionproducts and dicarboxylic acid (if present);

(ii) causing or allowing separation of the resulting reaction mixtureinto said liquid and solid phases and removing at least part of thesupernatant liquid phase from the reaction mixture; and

(iii) subjecting the mixture remaining after step (ii) to at least onefurther stage of hydrolysis.

It will be appreciated that the references to hydrolysis reaction stagesas used in the preceding paragraph are to be construed in the mannerpreviously referred to. Thus, for example, the further stage ofhydrolysis in step (ii) may be effected by combining the mixtureremaining after step (ii) to hydrolysis together with materialundergoing an earlier stage of hydrolysis and the reactions may beeffected either continuously or batchwise.

Usually precipitation of said intermediate reaction products in step (i)is effected by cooling the reaction mixture to a suitable temperature.

At least part or substantially all of the dicarboxylic acid formed inthe first stage hydrolysis reaction may be removed prior toprecipitation of said intermediate reaction products and/or prior tostep (ii). Thus, in step (iii), all or part of the dicarboxylic acidproduced in step (i) may be present, or the dicarboxylic acid may besubstantially absent as a result of being removed. Dicarboxylic acid soremoved may, if desired, be subjected to further hydrolysis separatelyand/or may be recycled to a different hydrolysis reactor to that usedfor further hydrolysis of the intermediate depolymerisation productsfrom which dicarboxylic acid is separated.

Preferably substantially all of said supernatant liquid phase is removedfrom the reaction mixture in step (ii).

Where the process involves more than two hydrolysis reaction stages, atleast one of the intermediate hydrolysis reaction stages may be operatedin the same way as the first hydrolysis reaction stage (ie. as specifiedin steps (i) and (ii) above).

In order to produce said reaction mixture capable of separating intodistinct liquid and solid phases after precipitation of the intermediatereaction products, the reaction of step (i) is conveniently carried outby limiting the amount of free diol present in the reaction. We havefound that control of the diol content limits the formation ofcomponents which tend to have a gelling-type action on the resultingreaction mixture. Such components, if present in substantial quantities,lead to the reaction mixture being of a sludge-like character andthereby inhibit separation of the reaction mixture into a distinctsupernatant liquid phase and a solid phase. Control of the diol contentcan be used to limit the amount of such inhibiting components presentfollowing each hydrolysis stage (apart from the final stage) thusleading to the production of a reaction mixture which can readilyseparate into distinct phases.

The diol content of the liquor may be controlled for example by alteringthe ratio of polyester/treated polyester to water used (if the polyesteris not pretreated by reaction with a diol as hereinbefore described);where such pre-treatment is used, control of the diol content can beeffected by removing a portion of the excess glycol prior to feeding thepre-treated polyester to the hydrolysis reactor--the amount of diolremaining following such removal will not normally be less than theamount that could have been realised from the starting polyester.

Because of the low solubility of dicarboxylic acids (particularlyterephthalic acid) in water, the hydrolysis reaction will usuallyproduce a two phase mixture at the reaction conditions (with the solidbeing substantially dicarboxylic acid). The dicarboxylic acid mayadvantageously be separated at a temperature of over 60° C. The liquorunder these circumstances may contain a high amount of solubledepolymerisation products. On cooling, the solubility of these productswill be reduced and, in the absence of diol control in accordance withthis aspect of the invention, may produce a sludge-like mixture in whicha clear supernatant forms only very slowly if at all. Separation of theliquor from the 'gelled' solid under these conditions is difficult. Thediol control feature of the present invention allows these solidoligomers to be easily separated from the liquor.

Usually step (ii) of the process will involve cooling of the reactionmixture to lower temperatures, typically 60° C. or less, eg. roomtemperature (ie. of the order of 20° C.) or even lower, in order tocause precipitation of the dissolved intermediate reaction products andseparation into the two distinct phases.

By operating all but the final stages of the hydrolysis in this manner,diol present in the reaction mixture can be readily separated from thesolid phase components which will largely consist of the dicarboxylicacid or species thereof and which can then be subjected to a furtherstage or stages of hydrolysis. Thus, the separation can be effected byroutine mechanical separation techniques such as decantation orfiltration.

The final stage hydrolysis reaction will typically convert substantiallyall of the depolymerisation products to dicarboxylic acid and diol.

In step (i) of the process, it is advantageous to remove any solidpresent in the hydrolysis mixture before precipitation of the bulk ofthe intermediate reaction products as this has been found to increasethe volume of supernatant liquor present after precipitaion of theintermediate reaction products. This solid (which is usuallysubstantially entirely dicarboxylic acid) may be blended with theintermediate reaction products after they have been precipitated andseparated from the liquor and the mixture hydrolysed in a furtherhydrolysis stage step (iii). However, as previously indicated, the solidmay also with advantage be kept separate, and treated separately tohydrolyse any remaining intermediate reaction products. In this case,the precipitated intermediate reaction products following step (ii) maywith advantage be returned to the original hydrolysis reactor andtreated with water along with fresh unhydrolysed material. In such ascheme, the recycled material will undergo a subsequent hydrolysis atthe same time as the fresh material undergoes its first stage hydrolysisreaction.

For the avoidance of doubt, it is to be understood that the variousreaction schemes described throughout may be carried out either on abatch or a continuous basis.

In one embodiment of the invention as applied to the depolymerisation ofPET in order to recover terephthalic acid and ethylene glycol (butapplicable to other polyesters in order to recover dicarboxylic acid andthe associated diol), the solid phase polyester (after being comminutedto a suitable particle size) is subjected to a glycolysis reaction atlow pressure (atmospheric or near-atmospheric) but elevated temperaturesufficient to produce a liquid phase medium containing the glycol andpolyester derivatives (primarily PET oligomers) resulting from the lowpressure glycolysis.

The glycolysis reaction is conveniently carried out at atmosphericpressure (or near atmospheric pressure, ie within several pounds persquare inch of atmospheric pressure) in the substantial absence ofmolecular oxygen. The glycolysis reaction preferably employs the sameglycol as that used in the production of the polyester, eg ethyleneglycol, and at a temperature in the range 140° to 280° C. (morepreferably 180° to 260° C., and most preferably at least 210° C., eg upto 230° C.). Usually the reaction is carried out at a temperature whichis more than 10° C. in excess of the boiling point of the diol.

Typically the reaction with the glycol is carried out using a PET:diolratio of at least 1:1, more preferably at least 1.5:1 and often at least2:1.

The low pressure glycolysis reaction is preferably carried out on acontinuous basis with solid phase PET and ethylene glycol being suppliedto the reaction concurrently with removal of liquid phase medium fromthe reaction and may be carried out at a temperature in excess of thenormal boiling point (about 196° C.) of the ethylene glycol used sincethe oligomers generated during the course of the reaction tend to havehigh boiling points thereby reducing the liquid vapor pressure andallowing the reaction to proceed without boiling off substantialquantities of the glycol. By carrying out the reaction at an elevatedtemperature compared with the normal boiling point of the glycol, thereaction may proceed more rapidly.

Liquid phase medium withdrawn from the low pressure glycolysis reactionis conveniently filtered at this stage using some form of mechanicalfilter to screen out particulates such as aluminum, lumps of PVC, paperetc commonly present in scrap/post-consumer PET. The liquid phase mediummay then be optionally treated to remove other impurities, for instanceby contact of the medium with activated carbon and/or an ion exchangeresin, in which case cooling of the liquid phase medium is effectedprior to such contact. Thus, where for example a technique involvingcontact with a material such as an ion exchange resin is employed,requiring the liquid phase medium to be cooled to a temperaturecompatible with the material employed in such technique, the liquidphase medium is typically cooled to a temperature within the range 50°to 130° C. (preferably 70° to 100° C.) prior to treatment by suchtechnique. The cooling may be effected either prior to filtration ofinsoluble impurities (eg aluminum, paper, PVC etc) from the liquid phasemedium or subsequent to such filtration.

Optionally the liquid phase medium containing glycol and low molecularweight depolymerisation products is subjected to a second glycolysisreaction at increased pressure (if necessary with added glycol) toenhance the degree of depolymerisation, preferably to form hydroxyethylene terepthalate compounds of the form: ##STR1## (which may betermed bis-hydroxy ethylene terephthalate (BBET) and mono-hydroxyethylene terephthalate (MHET) respectively). Usually the diglycoldepolymerisation products (such as BBET) will be the predominantcomponents and there may be little if any depolymerisation products withacid groups (such as MHET) present. Acid end groups are usually formedas a consequence of water being introduced into the glycolysis reactor(eg as a result of using damp glycol or PET). The more severe glycolysisreaction may be carried out at a temperature within the range 180° to260° C. (preferably 210° to 230° C.) and a pressure in the range 1 to 10bara (preferably 2 to 5 bara).

The liquid phase medium derived from the first glycolysis stage may bepumped continuously from the first stage to the second glycolysis stage.

Where two stages of glycolysis are used, both stages are convenientlycarried out in the substantial absence of molecular oxygen. Also, thefiltration and other treatment processes may be carried out at anysuitable point in the process, eg before or after the second stage.Typically, the filtration of insoluble contaminants may be effected at apoint intermediate the two stages by passage of the liquid phase mediumthrough a metal gauze or the like and the extraction of impurities usingactivated carbon, ion exchange resins or the like may be carried outafter the second stage, following cooling and pressure let-down of theliquid phase medium if necessary.

Following glycolysis and optional treatments such as impurityfiltration/extraction and glycol reduction, the liquid phase medium maybe cooled and collected for storage in a buffer tank or tanks.

Further details of the pre-treatment of the polyester by glycolysis aregiven in our copending Application of even date (also claiming priorityfrom UK Patent Applications Nos. 9313892.3 and 9313896.4), thedisclosure of which is incorporated herein in its entirety by thisreference.

Prior to hydrolysis of the liquid phase medium (following glycolysisreaction in one or two stages), the medium is conveniently processed toreduce its glycol content, eg by flashing in a suitable vessel, therebyrequiring less water in the subsequent aqueous hydrolysis step togenerate any desired number of acid end groups. The glycol content willcomprise both glycol added as reactant (in both glycolysis stages whereapplicable) and that generated in the course of the glycolysis and thereduction process will usually result in removal of a major part of theglycol. The removal of glycol, eg by flashing, is preferably effected,eg at low pressure, to avoid excessive rise in viscosity andrepolymerisation of the hydroxy ethylene terephthalate compound(s).

The hydrolysis reaction is carried out in at least two stages and maycomprise a substantially neutral aqueous phase hydrolysis, iesubstantially in the absence of added acids or alkalis.

In the first stage of the hydrolysis reaction, the treated polyesterobtained from the above described two stage glycolysis process iscontacted with aqueous medium in a vessel such as an autoclave at arelatively low reaction temperature (certainly less than 300° C.,typically in the range 190° to 240° C. and preferably about 200° toabout 220° C.). Hydrolysis at temperatures lower than 190° C. may beemployed if desired but the reaction time then has to be increasedsignificantly.

The quantity of water added in the first stage is less than thatrequired for total conversion of the polyester/treated polyester toterephthalic acid. Following the first hydrolysis reaction, at least amajor part of the liquor (which will contain significant amounts ofglycol) is separated from the terephathalic acid and acid/glycolreaction products. If necessary, more water is then added to theremaining material and further hydrolysis is effected under conditionsas specified above to yield terephthalic acid as an end product.Alternatively, the second hydrolysis may be an intermediate reactionfollowed by a further stage or stages of hydrolysis. For instance, thesecond stage hydrolysis may be carried out using less water than isnecessary to achieve substantially total conversion of the materialremaining after the first stage hydrolysis to terephthalic acid. In thisevent, following the second stage hydrolysis, the liquor is againremoved and the material yielded by the second stage hydrolysis(comprising terephthalic acid and species thereof) is subjected to afurther hydrolysis reaction.

The advantages of this approach are that less water in total is requiredin order to achieve a given degree of hydrolysis, and the liquorrecovered from the first stage has a much higher glycol content whichmakes glycol recovery easier.

The first hydrolysis stage is desirably carried out in such a way as toproduce a reaction mixture which is amenable to separation into twodistinct phases following cooling to precipitate any terephthalic acidspecies. If desired, free dicarboxylic acid may be recovered prior toprecipitation of the bulk of the dissolved oligomers but, in this case,it is advantageous not to cool so far as to precipitate the bulk of thedissolved oligomers.

Especially desirable is to control the glycol level so that aftercooling to between 70° C. and 200° C. the solid present is at least 50%preferably at least 70% and especially at least 90% terephthalic acid.This solid is normally present in an easily seperable form (a clearsupernatant liquid above a rapidly settling solid). After removal ofthis solid, the residual liquid may be cooled further in order toprecipitate the bulk of the dissolved terepthalic species (typically toabout 25° C.). It is especially desirable to control the glycol level inthe original liquor so that, on cooling, the solid which is precipitatedin this stage leaves a clear supernatant. This liquor component of thereaction mixture, being a well-defined supernatant liquid, can bereadily removed by decantation or mechanical filtration.

The liquor so removed may, given time, tend to throw a precipitate offurther slow crystallising species containing terephthalic acid whichmay be recovered, for example in a settling tank, and following settlingthe lower portion of the liquor/solid can be recycled for hydrolysistogether with subsequent batches of polyester/treated polyester. Theupper portion from the settling tank may be treated in order to effectglycol and water recovery.

One advantage of this method of removing the liquor comprising theliquid phase component of the hydrolysis reaction mixture is that if thehydrolysis reaction is not conducted in such a way as to achieveeffective separation of the reaction mixture into two distinct phases, aprocess such as distillation would be needed to recover the liquor fromthe hydrolysed solid and, in that event the water will come off firstleaving glycol only and while some of the glycol will be removed, therest will either remain on the solid or will react with the terephthalicacid thereby reversing the hydrolysis reaction.

Another advantage is that the liquor separated from the solid containsany soluble contaminants present in the scrap PET not removed byprevious purification steps.

However the liquor is removed, it is likely that it may be recycled byeither distillation of the first hydrolysis liquor or storing liquorfrom the second or third hydrolysis for use as the aqueous phase in apreceding hydrolysis stage. This latter possibility arises since eachsucceeding stage of hydrolysis will produce a liquor having a lowerglycol content (and hence a more water rich liquor) which can berecycled to a preceding hydrolysis stage.

After hydrolysis is complete, the terephthalic acid is recovered bysuitable filtration and drying (if required--ie. drying may not benecessary if the recovered terephthalic acid is to be blended withterephthalic acid derived from other sources such as the liquid phaseoxidation of p-xylene). Advantageously the recovery of the terephthalicacid includes one or more washing steps, using the same or differentwash liquors for each step where multiple washing steps are employed, toremove particular species of organic impurities (for instance to ensurefood contact approval), especially water insoluble impurities, to reducethe water content and improve the product color.

The washing step(s), or any one or more of them, may be carried outusing heated wash liquor.

Filtration of the terephthalic acid is conveniently carried out by meansof a belt filter. Following filtration the terephthalic acid filter cakemay be transported on the belt filter through one or more washing stagesin which it is washed with a wash liquor or more than one wash liquor,the wash liquor(s) being drawn through the belt filter to leave a washeddeposit which may then be dried in any suitable manner. Acetone is aconvenient washing liquor since it may serve all of the purposesmentioned above, ie removal of organic species, improvement of productcolor and drying. The use of acetone as a wash liquor in this manner, iewashing of terephthalic acid particularly terephthalic acid recovered byhydrolysis of polyester or pre-treated polyester as referred to herein,constitutes a further aspect of the invention which may be consideredadditional to or separate from other aspects of the invention disclosedherein.

The terephthalic acid recovered from the process may be re-used in theproduction of polyesters, if necessary after the terephthalic acid hasbeen subjected to a purification process such as that conventionallyemployed in the production of pure terephthalic acid. Thus, for example,the recovered terephthalic acid may be dissolved to form an aqueoussolution which is then contacted with hydrogen in the presence of anoble metal catalyst (eg palladium and/or platinum supported on an inertsupport such as carbon) at a temperature within the range 250° to 350°C. and hydrogen partial pressure of 5 to 25 bara. Alternatively theterephthalic acid may be purified by recrystallisation from solution.

As mentioned previously, it can be advantageous to effect hydrolysis insuch a way that a substantial proportion of the terephthalic acid isproduced in the solid phase during the course of the reaction. Theformation of terephthalic acid in the solid phase during the hydrolysisreaction results in a relatively small particle size and also allowsparticle size to be controlled at this stage. More specifically,particle formation is preferably controlled in such a way that theparticles of solid phase terephthalic acid particles forming during thehydrolysis reaction are of rounded shape, desirably such that at least90% of the particles of the recovered solid phase terephthalic acid aresufficiently small to pass through a sieve having a grid size 2 mm,preferably 1 mm, more preferably 800 microns, and especially 500 micronssquare.

Thus, by controlling the particle size during the hydrolysis reaction,it becomes possible to achieve a desired particle size and distributionconsistent with the requirements imposed by subsequent processing of theterephthalic acid product, without the necessity for a separateprocessing vessel (eg crystalliser) for treating the terephthalic acidin order to obtain the desired particle size and distribution. Variousways of controlling particle size can be contemplated such as control ofthe temperature gradient within the reaction vessel and/or the provisionof surfaces which promote formation of the desired particle shape andsize. One particularly effective control techique is to effect agitationof the reaction mixture during hydrolysis, for example by means ofstirring.

Agitation may be continued after the hydrolysis reaction has beencompleted and during cooling of the reaction mixture so as to promotecrystallisation of terephthalic acid which has remained in solution inthe form of rounded particles (as opposed to needle-shaped particleswhich may be up to 1 cm or above in length as tends to happen if thesolution is allowed to cool naturally).

Preferably therefore, the reaction mixture is suitably agitated duringheating. By suitably controlling particle size formation from thereaction mixture, for instance by agitation of the reaction mixture, itis possible to secure that at least 90% of terephthalic particlesrecovered are of rounded shape capable of passing a sieve having asquare grid size of 2 mm (more preferably 1 mm and even more preferably500 microns), as opposed to needle-shaped particles, which isadvantageous when the particles are subsequently slurried with alkyleneglycol in the course of PET production since particle packing density isof importance in this respect.

It will be understood that, where in the processes disclosed abovereferences are made to terephthalic acid, such processes may also beapplied to such other dicarboxylic acids and diols as are used in theproduction of polyesters.

The invention will now be described further by way of illustration onlywith reference to the following Examples.

EXAMPLE 1

1000 g of pulverised PET and 2000 g of MEG were introduced into a 4liter capacity autoclave fabricated from a Hastalloy material and fittedwith a nitrogen purge for producing an inert gas atmosphere within theinterior of the autoclave. The components were mixed by means of a smallturbine stirrer and were heated to 200° C. for 5 hours with 2000 g MEGto produce a liquid phase product. The product was found to have Mn=305,Mw=367. The liquid was passed hot through a 300 micron brass sieve. Theliquid was then distilled at atmospheric pressure, the glycol boilingoff at up to 200° C. 640 g of glycol was removed in this way so thatless water is needed in the subsequent hydrolysis reaction to facilitateeasy separation of liquor from precipitated oligomers. At this stage, Mnof the product was found to be 341, indicating that somerepolymerisation had occurred.

Following removal of glycol, 1500 ml distilled water was added to theproduct and the mixture was heated to 203° C. in an inert gas atmospherefor 2 hours. The amount of water used in this stage is significantlyless than that required to effect total conversion of the polyester toterephthalic acid.

The reaction mixture was then cooled to 70° C. and was found to form awell-defined two phase system comprising a clear yellow/brownsupernatant liquor and a solid phase residue. Substantially all of thesupernatant, amounting to 1200 ml of the liquor, was sucked out using adip pipe fitted with a fibrous felt filter. The resulting damp residuewas found to comprise 75% by weight solid of which about 97% by weightwas terephthalic acid. Of the supernatant liquor removed, 1000 ml wascooled to room temperature (about 23° C.) and 100 g of solid materialwas found to have precipitated out. This solid material was readilyfiltered from the liquor and, on analysis, was found to comprise almost100% MET.

Following removal of the liquor from the two phase reaction productresulting from the first stage hydrolysis, a second stage hydrolysis wascarried out by adding 667 ml of water to the vessel contents (comprisingthe terephthalic acid-containing solid phase residue), and the mixturewas heated to 200° C. for 2 hours in an inert gas atmosphere to completethe hydrolysis. A white powder product was recovered following coolingwhich was found to comprise approximately 99% terephthalic acid byweight.

Normally, using a single stage hydrolysis without pre-glycolysis, itwould be necessary to use about 4 litres of water to obtain a yield of99% from 1000 g of PET. If pre-glycolysis is used, an amount of waterconsiderably in excess of 4 litres would be needed to secure the sameyield. In contrast, it will be seen that the process of the invention asillustrated by Example 1 can be carried using a substantially reducedamount of water.

EXAMPLE 2

300 g glycol was heated with 700 g PET flake under an inert gasatmosphere in an insulated glass vessel fitted with a nitrogen purge, athermocouple and a condenser. The vessel was open to atmosphere and wasvapor-jacketed, the vapor being supplied by boiling dodecanol (bp. 260°C.). The liquid finally reached 226° C. and was held at this temperaturefor 4 hours. After this time Mn and Mw were found to be 431 and 583respectively. The liquid was drained out of the vessel via a tap in thebase, the tap being provided with a filter in the form of a plug ofglass wool. Because only a small amount of glycol had been added to thePET there was no need to remove the glycol prior to hydrolysis.

883 g of the glycolysed material was transferred to the autoclave ofExample 1 and 871 g of distilled water were added. The mixture washeated with stirring for 2.5 hours at 206° C. The amount of wateremployed constituted about 1/6th of that necessary to effect totalconversion of the polyester to terephthalic acid. The stirrer rate was400 rpm (small turbine stirrer). After 2.5 hours, the pot contents werecooled to 70° C. The product was a white powder with a clearyellow/brown supernatant liquor. The supernatant layer (about 1000 ml)of liquor was removed by suction through a dip pipe fitted with afibrous felt filter. The dried solid was found to comprise 95.2%terephthallic acid by weight.

A further 1240 g distilled water was then added to the remaining solidin the vessel and the mixture was reheated to 200° C. for 2 hours. Theproduct was filtered hot and rinsed in acetone and dried. A white powderwas recovered containing 98.3% by weight terephthalic acid, less than1.5 ppm of each of Na, Mn and Co and 4.7 ppm Sb.

EXAMPLE 3

A sample of post-consumer PET was glycolysed for 2 hours at 260° C.using two parts by weight glycol to one part PET. The conductance of acell containing the material at 80° C. was 2.0 μS. The material waspassed (at 80° C.) through a column containing a bed of cation ionexchange resin followed by a bed of anion ion exchange resin. Theconductance of a cell containing the material produced from the ionexchange resins at 80° C. was 0.2 μS, demonstrating that a significantproportion of the free ions had been removed. Elemental analysis showedthe starting material and treated material to contain the elements atthe levels given below:

    ______________________________________    Element    Starting Material                           Treated Material    ______________________________________    Sb         45 ppm      1.5 ppm    Cl         79 ppm      11 ppm    ______________________________________

This Example illustrates the advantage obtainable by liquefying the PETpolyester prior to hydrolysis, ie the cooled liquid phase medium can betreated using ion exchange resins to remove certain impurities.

EXAMPLE 4

A sample of PET which had been deliberately contaminated withbenzophenone was hydrolysed using the process described in Example 2.After contamination, the PET contained 1.64% benzophenone by weight. Theresulting terephthalic acid produced by the hydrolysis containedapproximately 0.11% benzophenone by weight. 20 g the terephthalic acidproduct was treated by Soxhlet extraction using 200 g of acetone. Theacetone obtained following the extraction procedure was found to contain0.014% benzophenone by weight. No benzophenone could be detected in theterephthalic acid following treatment thereof with acetone indicatingthat any residual benzophenone content was less than 100 ppm.

It will be appreciated that, whilst the invention has been describedhereinbefore with reference to the processing of PET to recoverterephthalic acid and glycol, similar process steps may be employed inthe case of other condensation polyesters such as polyethylenenaphthalate.

We claim:
 1. A process for depolymerizing a scrap or post-consumer solidcondensation polyester selected from polyalkylene naphthalate andpolyalkylene terephthalate to recover the constituent dicarboxylic acidand diol therefrom which comprises:(a) comminuting the solid polyester;(b) reacting the comminuted polyester with a diol in the absence ofwater at a polyester:diol ratio of at least 1:1 by weight and at atemperature within the range of 140° to 280° C. to produce a liquidphase medium; (c) removing diol from the liquid phase medium; (d)subjecting said liquid phase medium from step (c) to a first hydrolysisreaction which comprises contacting said liquid phase medium with waterat a temperature in the range of from 190° to 240° C. whereby a firstportion of the constituent dicarboxylic acid present in the liquid phasemedium precipitates; (e) separating from the reaction medium formed instep (d) at least part of the diol present therein and optionallyremoving from the reaction medium at least some of said first portion ofconstituent dicarboxylic acid precipitate; (f) subjecting the reactionmedium remaining after step (e) to at least one further hydrolysisreaction which comprises contacting said reaction medium with water at atemperature in the range of from 190° to 240° C. whereby at least asecond portion of the constituent dicarboxylic acid present in theliquid phase medium from step (b) precipitates; and (g) optionallywashing and filtering the precipitate from steps (d) and (f) to recoverthe constituent dicarboxylic acid.
 2. The process as claimed in claim 1in which the reaction in step (b) is carried out at a temperature whichis in excess of 10° C. above the boiling point of the constituent diol.3. The process as claimed in claim 1 in which reacting the comminutedpolyester with a diol in step (b) comprises (i) reacting the comminutedpolyester with a diol at atmospheric pressure in a first low pressurestage, and then (ii) reacting the mixture resulting from step (i) with adiol at a second high pressure stage in the optional presence ofadditional diol than was present in step (i), with the proviso that thediol used in steps (i) and (ii) is the same diol used originally as amajor component in the production of the polyester and steps (i) and(ii) are carried out in the substantial absence of molecular oxygen.